Amino acids must be supplied to the fetus by the placenta to meet fetal metabolic needs for growth and catabolism. The overall objective of this research is to understand the mechanism and regulation of amino acid transfer from mother to fetus. Work is directed towards characterization of the pathways by which the human placental syncytiotrophoblast takes up amino acids from maternal blood and study of their mechanism and regulation. Two approaches will be used: 1) Placental tissue will be incubated in vitro and data analyzed kinetically for comparison with the needs of the fetus and in vivo placental transfer. Previous investigations of this project have identified three neutral amino acid transport systems, and demonstrated that one of these can apparently be regulated by two different mechanisms and these studies will be extended during this period. 2) An isolated syncytiotrophoblast plasma membrane preparation will be used to study amino acid transport. Previous investigations of this project have demonstrated the feasibility of this approach and shown the suitability of the preparation for structural and other functional studies which can be pursued in association with the investigations of amino acid transport.